AN investigation was demanded yesterday into the soaring cost of five North-East road schemes - up a staggering 56 per cent.

The Liberal Democrats said ministers should ask the independent National Audit Office (NAO) to examine the escalating bill for local taxpayers.

The roads budget should be frozen until the NAO has established if the construction industry was "getting away scot-free" with pushing up costs, the Lib Dems said.

But the Department for Transport (DfT) insisted yesterday that local authorities were responsible for any cost overruns once the schemes had gained Whitehall approval.

The five schemes were expected to cost a total of £47.3m when given the go-ahead between 2000 and 2004, but the bill is now set to reach £73.7m.

The schemes are:

* Darlington Eastern Transport Corridor, up from £5.7m to £12.1m (112 per cent).

* Sunderland Southern Radial Route, up from £17m to £28.7m (69 per cent).

* A1056 Northern Gateway, North Tyneside, up from £9.9m to £14.3m (44 per cent).

* A688 Wheatley Hill to Bowburn Link, Durham, up from £6.6m to £9.2m (39 per cent).

* Pegswood Bypass, Northumberland, up from £8.1m to £9.4m (15 per cent)

The increases do not take inflation into account but - at only about 2.5 per cent a year - that would not explain a rise of 56 per cent.

Tom Brake, the Lib Dem transport spokesman, said: "There seems to be one rule for public transport and another for road building.

"Whereas the Government is dropping plans for new trams based on the argument that costs are escalating, the road construction industry appears to be getting away scot-free.

"There should be an immediate freeze on the roads budget until we get to the bottom of what is driving these cost rises."

The Darlington corridor scheme will provide access to the Central Park regeneration project, easing congestion and freeing up further land for development in the east of the town.

Meanwhile, work began last September on the Sunderland Radial Route, connecting the city centre with the A19 north of Seaham, County Durham.

All five schemes are being funded as part of local authority transport plans, through which more than 60 projects, costing at least £5m, have got the green light.

They were estimated to cost £1.21bn at the point of approval, but have now risen by 45 per cent to a combined bill of £1.75bn.